Ukraine's SBU security service confirmed that Vladimir Tsemakh, a fighter with Russian-backed separatists considered a key witness in the downing of flight MH17, was also released as part of the swap.

Russia refused to proceed with the exchange unless Ukraine agreed to hand over the key witness, a senior Ukrainian official said.

Anticipation had been building for days for the exchange, which involved weeks of behind-the-scenes negotiations.

Relations between Kiev and Moscow were fiercely damaged in 2014 when Russia annexed Crimea and Moscow backed separatists in the eastern industrial regions of Donetsk and Lugansk.

Fighting there has claimed more than 13,000 lives over the past five years.

Mr Zelensky's election in April has raised hopes that the stalled peace process could be revived.

The comedian-turned-politician vowed during the campaign to have Ukrainian prisoners in Russia returned and has said ending the conflict with Russia is his top priority.

Russian President Vladimir Putin said this week that the exchange would be "a huge step towards normalising relations" with Kiev.

German Chancellor Angela Merkel hailed the swap as a "sign of hope" and called for the implementation of a 2015 ceasefire deal that Paris and Berlin helped broker.

"This exchange of prisoners between Russia and Ukraine is a sign of hope ... it's worth continuing the hard work to implement the Minsk accord," Ms Merkel said in a statement put out by her spokesman Steffen Seibert on Twitter.

US President Donald Trump welcomed the prisoner exchange as a potential step towards peace.

"Russia and Ukraine just swapped large numbers of prisoners. Very good news, perhaps a first giant step to peace. Congratulations to both countries!" Mr Trump tweeted.

Sailors, filmmaker, journalist

The release of Mr Sentsov will be seen as a major victory for Kiev.

The 43-year-old was Ukraine's most famous political prisoner and the subject of a star-studded international campaign calling for his release.

He was arrested in 2014 and had been serving a 20-year sentence in an Arctic penal colony for planning "terrorist attacks" in Crimea.

"I thank all the people who have fought for us," Mr Sentsov said at the airport in Kiev.

The sailors, including two members of Ukraine's SBU security services, were detained last year when Russia seized three Ukrainian vessels off Crimea.

Among those handed over to Russia was Vyshynsky, a journalist at Russia's RIA Novosti state news agency who was facing charges of "high treason" but was released on bail late last month.

The release of Tsemakh, an alleged air defence specialist for pro-Russian separatists, prompted concern from the Netherlands as he is considered a person of interest in the MH17 case.

The Malaysia Airlines passenger plane travelling from Amsterdam to Kuala Lumpur was shot down by a Russian-made BUK missile in 2014 over eastern Ukraine, with the loss of all 298 people on board.

France has urged renewed efforts to resolve the conflict, calling for a summit of the leaders of Ukraine, Russia, France and Germany later this month.

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